Bass Rock
Bass Rock 2001 © Dr. J.B Nelson
Dense stands of tree mallow
Dense tree mallow stands on Craigleith

Invasives, Biodiversity & Climate Change

Understanding the spread of tree mallow

Whereas tree mallow has been on the Bass Rock for over 300 years, the species has rapidly expanded during the last 15 years in the Firth of Forth region, perhaps helped by climatic change. There is therefore an urgent need to understand how climatic factors influence the success of this tall invasive plant that is responsible for the suppression of puffins.

Not only may puffins be up against climate change via the expansion of tree mallow, warmer sea water in winter is also known to reduce the breeding success of seabirds. The majority of seabirds in the North Sea feed on sandeel. Sandeels spawn in winter and fewer larval fish grow and appear to survive in warm winter waters, potentially reducing food availability for seabirds that will also reduce their breeding success. (Source: Frederiksen, M. and Wanless, S. in prep).

Puffin burrow with tree mallow seedling in the foreground

The activities of puffins also assist in the establishment of tree mallow. Tree mallow needs a fertile soil to establish itself, and soils of seabird islands are nitrogen rich as a result of the guano and ammonia from faeces of seabirds themselves. On Craigleith, tree mallow seedlings grow predominantly in gaps in the vegetation that are created by puffins through their digging to create entrances to burrows, or in areas where puffins land, fly off or frequently walk.

Puffin burrow with tree mallow seedling in the foreground
 

Practical solutions need to be developed and tested to restore biodiversity on those islands currently dominated by tree mallow and to prevent biodiversity loss on islands where establishment is at a less advanced stage.